1. Field of the Invention
This invention relates generally to radiographic systems and is concerned more particularly with an X-ray test system for evaluating focal spots in X-ray tubes.
2. Discussion of Prior Art
An X-ray tube usually comprises an evacuated envelope wherein an electron emitting cathode is disposed for electrostatically beaming electrons onto an aligned focal spot area of an anode target surface. Generally, the focal spot area of the target surface is sloped in the direction of a radially aligned, X-ray transparent window in the tube envelope. Accordingly, X-rays generated in the target material by the impinging electrons radiate from the focal spot area and pass in a beam through the X-ray transparent window of the tube. As a result, the X-ray beam appears to be emanating from a radial projection of the focal spot area on the sloped target surface. Thus, the resolving power of the X-ray beam may be maximized by maintaining the projected focal spot area as small as possible such that is approximates a point source of X-radiation.
In operation, the X-ray beam may be directed through a selected portion of a patient and be modified by the X-ray absorption properties of the internal structure therein. As a result, the modified X-ray beam may convey an X-ray image of the internal structure to an aligned receptor, such as an X-ray film, for example, which produces a visible shadow image of the X-ray image conveyed by the beam. However, it has been found that internal structure comparable in size to the projected focal spot appears blurred in the output visible image, and may even appear as doublets when higher magnification techniques are employed. Thus, for fine detail structure, the projected focal spot may not function as a point source, and the resulting X-ray beam may have insufficient resolving power for sharply defining the image of the structure. Consequently, the size of the projected focal spot may contribute to geometrical unsharpness which constitutes a limiting factor in many radiological techniques, such as magnification of small blood vessels in angiography, for example.
As a result, it has become common practice when describing a radiological procedure to specify the geometry and the nominal size of the focal spot, which is equivalent to defining a tolerable geometric unsharpness in producing the visible image. Accordingly, the National Electrical Manufacturers Association (NEMA) has advocated the standardization of focal spot measurements by the use of two methods, namely, the pin-hole camera method and the star test plate method. The pin-hole camera method has not been found satisfactory for wide spread use because of the limited amount of X-radiation passed through the pinhole and the associated requirement for high electron current in the X-ray tube being tested. The star test plate method has proved to be more practical for routine use and also may provide some information on energy distribution in the focal spot. However, due to the complex resolution pattern produced by the star test plate, measurements for determining focal spot size are not readily obtained with the ease and accuracy desired.
Therefore, it is advantageous and desirable to provide an X-ray tube test system having means for readily evaluating focal spots in X-ray tubes.